The permissible exposure to airborne concentration of asbestos fibers is expressed in terms of number of fibers longer than 5 micrometers per unit volume of air sampled. While there is general conviction that the fibrogenic effect of asbestos is physical size dependent, definitive studies have not been performed because of the lack of suitable techniques for size-classifying asbestos samples. Grinding of fibers not only changes the length, but also diameter and crystallinity. In the present proposal, a new opposing-jet impaction technique, first designed for the size-classification of spherically-shaped particles, developed by the principal investigators, will be further developed for the classification by length of asbestos fibers. In this technique, the fibers are aligned parallel to the streamlines in the airjet of the first impaction stage and perpendicular to the streamlines in the airjet of the second stage. By opposing the asbestos-laden airjet by a clean airjet of the same magnitude, the fibers are classified into two effluent streams depending on their length, diameter and alignment in the flow-field. Length-classified fibers of glass and of different asbestos types will be administered to animals intra-tracheally to study relative early fibrogenic effects on the respiratory tract and by gavage to study relative effects on biochemical parameters in cells of the gastrointestinal tract.